This invention relates to a process for the solvent refining of coal wherein coal is liquefied by subjecting it to a hydrogen donor solvent (hereinafter referred to as "solvent") in the presence of a hydrogen-rich gas at elevated temperatures and pressures to produce solid and liquid products. This process is referred to in the art as SRC-I, solvent refined coal having the acronym "SRC".
In this process, following solvation, the products are separated into gaseous material, distillate fractions and vacuum distillation bottoms. The vacuum distillation bottoms, which contain entrained mineral matter and unconverted coal macerals, are separated in a deashing step. From the solids removal step there is recovered a stream of coal products which are free of ash minerals and unconverted coal and which are essentially low in sulfur content, such that this material is ideally suited for combustion in environmentally acceptable operations.
The SRC-I pilot plants at Wilsonville, Ala. and Fort Lewis, Wash. have been operated with a coal liquefaction reactor (also known as the dissolver) preceded by a preheater. The coal liquefaction reactions take place to some extent in both these vessels. A slurry of coal in recycled solvent under hydrogen pressure is passed through the preheater where its temperature is raised from ambient to a temperature in excess of 750.degree. F. The heated slurry is passed to the reactor whereat the reaction of the hydrogen gas, the coal and the solvent take place at temperatures in excess of 780.degree. F. and pressures in excess of 1,000 psia, the liquefaction reactions including desulfurization, solvent production, solvent rehydrogenation, etc.
So long as hydrogen gas is present, the forward rate of reaction to produce asphaltenes an oils from dissolved coal is greater than the retrograde repolymerizations which lead to the formation of coke and preasphaltenes from the lower molecular products. However, at the exit of the reactor it is necessary to separate the gas phase containing hydrogen from the slurry phase containing the soluble coal products and solid residues. This separation is carried out as a first stage in the separation of the reaction products. In the absence of hydrogen gas, it is known that coke formation may occur and preasphaltenes are formed by repolyerization. These undesirable reactions are increased by increased temperature and residence time.
The problem of coke formation in the reactor effluent separator when operating at or close to reactor temperature has been observed at the Wilsonville pilot plant. At Wilsonville, coke formation in the outlet separator was observed when operating at 800.degree. F. while previous operation at temperatures below 780.degree. F. did not encounter this problem.
One method which has been used to prevent retrograde reactions is to directly cool the total coal reactor effluent either by heat exhange or by quenching. These procedures have been used in the pilot plant operations at Wilsonville and Fort Lewis. More specifically, the coal liquefaction reactions occur at a temperature in the range of 800.degree.-880.degree. F. and the three phase effluent is cooled to a temperature generally below 780.degree. F. sufficiently low to prevent coke formation prior to phase separation. In the design of the SRC-I Demonstration Plant the cooling of the effluent is effected using recycled solvent. Thus, the prior art method involves cooling the total reaction product stream leaving the reactor and is inherently inefficient.